The present invention relates to an improved water impervious rubber or plastic insulated power cable.
A power cable insulated with rubber or plastic has a basic structure as shown in FIG. 1. Referring to FIG. 1, a conductor shield 2, an insulation layer 3, an insulation shield 4, a metal shield layer 5, and a jacket 6 are formed in the order named around a conductor 1.
In the conventional cable of this type, a water impervious layer 7 is formed below the jacket 6 so as to prevent water infiltration from outside the cable for application in the fields of communication and power transmission cables. The water impervious layer 7 comprises an aluminum laminated tape, each surface of which is coated with polyethylene, and is bonded to the jacket 6 by the heat generated during extrusion coating of the jacket 6. However, the polyethylene in the aluminum laminated tape (hereinafter referred to as "water impervious tape") used in the water impervious layer 7 has a high insulation characteristic. For this reason, when a voltage is induced in the metal (aluminum) layer of the water impervious layer 7 or a surge voltage is applied to the cable, a potential difference is established between the metal layer of the water impervious layer 7 and the metal shield layer 5 on the cable core. When the polyethylene layer of the water impervious tape breaks down because of such a potential difference, a pinhole is formed in the metal layer of the thin water impervious tape by an arc generated at the time of breakdown. Then, the water impervious layer 7 loses its water-resistant and water-impervious characteristics. The cable insulation may then be degraded by water or chemicals which infiltrate through such a pinhole in the water impervious layer 7, resulting in lower performance or shorter service life of the cable.